The present invention relates generally to an improved camera body and to a method for manufacturing the same from plastic, and it relates more particularly to an improved camera body which is highly adapted for use with an interchangeable objective lens.
Hitherto, it has been a general practice to use a light alloy casting such as an aluminum alloy for a camera body. However, a camera body of a light alloy casting is costly and still heavy in weight, and thus there is a great demand for a camera body which is both light in weight and low in cost.
To meet this demand, many attempts have been made to produce a camera body of plastic, particularly cameras of a medium size, which are equipped with a so-called lens shutter. These structures have been already employed in the field of a camera industry. However, in the case where the camera body is made only of a plastic, the following shortcomings are experienced:
(a) The coefficient of thermal expansion of a plastic is extremely large, as compared with those of light alloys such as aluminum alloys. Even glass-fiber-reinforced plastic affords a coefficient of thermal expansion twice as large as that of aluminum. This results in the failure to achieve the dimensional accuracy required for a camera, and deformation due to the cooling following molding, with the resulting impaired flatness of the film abutting surface.
(b) Recently, cameras have been equipped more or less with electric circuits. However, the cameras made of plastic are not suited to a body-grounding system which is an essential factor in simplifying the wiring of such an electric circuit.
These shortcomings are common in cameras having plastic bodies. The above shortcoming (a) particularly leads to the lowering in the performance of a high-class camera of the lens interchangeable type, such as a single lens reflex camera and the like. For instance, assuming that the thickness of the plastic portion of a camera body is about 25 mm, and that the serviceable temperature range of the camera is from - 20.degree. C. to + 50.degree. C., then there occurs in the camera a dimensional error of the order of at least 60 .mu.m. This dimensional error far exceeds the allowance for a camera having a large aperture in an interchangeable lens having a small depth of focus, because the above dimensional error has a close bearing upon the so-called flange back (the distance between the mount surface of a camera body for an interchangeable lens and the film plane). Meanwhile, in the case of a camera having an interchangeable lens, consideration should be given to a heavy interchangeable lens such as a telephoto lens. In case the camera body is made of plastic in its entirety, then its strength is less than that of a light alloy camera body, and thus such a camera is not suitable for use with a large-size interchangeable lens and imposes a limitation on the useable interchangeable lenses. To avoid this shortcoming, it has been proposed to use a glass-fiber-reinforced plastic. However, desired machinability can not be achieved and thus a difficulty arises in the treatment of the film-abutting surface on which the film pressure plate is abuttable. (This will hereinafter be referred to as the film-abutting surface). Accordingly, such a camera finds no application for a large aperture lens having a small depth of focus.
These shortcomings restrict the progress of the manufacture of cameras having camera bodies made of plastic, particularly the manufacture of those cameras having interchangeable lenses. A composite body consisting of a metal block and a plastic body is described in the Japanese laid-open patent publication No. Sho 51-6719, as a proposed solution to the aforesaid shortcomings. In the aforesaid composite camera body, a front metal plate, to which an interchangeable lens may be attached, and a metal block having an attaching surface for the aforesaid metal plate and an aperture delineating or defining an exposure frame, and an abutting surface for the film, are made of a light alloy, and the metal block thus prepared is used as an insert for the plastic-molding of the body proper of the camera, to which a part of the film feeding mechanism may be attached. This structure reduces not only variation in flange back due to temperature change but also deformation in respective portions of the body proper due to cooling, as in the case of a camera body made of a light alloy, and in addition allows the machining of the abutting surface of the film with a high accuracy, because of the satisfactory machinability of the metal block.
However, the metal block described in the Japanese laid-open patent publication No. Sho 51-6719 consists of a rear metal plate having a film abutting surface and an aperture, and bent portions which are provided respectively at the right edge, the left edge and the lower edge of the rear metal plate. These bent portions are provided independently of one another and hence barely reinforce the rear metal plate. Accordingly, upon plastic molding, when the body proper of the camera body is shrunk or contracted to a large extent due to cooling, then the rear metal plate is somewhat deformed, as in the case where external stresses are applied to a sheet or flat plate from rightwards and leftwards and upwards and downwards. In addition, it is a common practice to prepare the film abutting surface of the rear metal plate by milling, after the molding of a body proper of a camera body. However, if a large stress is applied thereto widthwise, i.e. in the direction perpendicular to the rear metal plate upon machining, then the rear metal plate is deformed, resulting in impaired flatness of a film which abuts film rail surfaces.
Furthermore, upon the plastic molding of the body proper of a camera body by using a metal block as an insert, the metal block is fitted in dies for a body proper, and then secured in position by mating dies or die halves, followed by pouring molten thermoplastic into the dies. However, with a metal block such as described in the Japanese laid-open patent publication No. Sho 51-6719 in which the metal block is used as an insert, bent portions thereof directed to the right and left and upwards and downwards are readily deformed due to external stresses, so that the mere mating of dies would not lead to reliable location of metal block therein.
In general, an aperture defining an area of an image of an object, which is to be projected onto a film, is located in the center of the camera body. In addition, provided on the opposite sides of the aperture are (i) a box shaped portion, to which part of a shutter charging mechanism and a film feeding mechanism are attached, and which includes a cavity housing a film take-up spool and a sprocket therein, and (ii) another box shaped portion, to which a film unwinding mechanism is attached and which has a cavity housing a film cartridge therein. A camera body having the aforesaid metal block and plastic-molded body proper is no exception to this. As shown in the Japanese laid-open patent publication No. Sho 51-6719, a frame aperture defining an area of an image of an object, which is projected onto the film, is positioned in the metal block substantially in its center, and the aforesaid box shaped portions are formed as part of body proper made of plastic, with the metal block being incorporated therein as an insert, and the-box shaped portions are positioned on the opposite sides of the metal block, while being continuous with each other through the medium of interconnecting portions of the body proper of a camera body, which portions extend along the upper and lower edges of the metal block, respectively.
Moreover, in the case where a camera body is plastic-molded, with an aluminum piece used as an insert, a molten light alloy is generally poured into such a cavity in a die, which corresponds to one of the right-hand and left-hand box shaped portions, and then the molten light alloy thus poured is directed through cavities corresponding to interconnecting portions into a cavity corresponding to the other box-shaped portion. A framing aperture defining an area of an image of an object, which is projected onto a film and a film abutting surface are prepared separately after the molding. However, the process for pouring a molten plastic into a cavity corresponding to one of the box shaped portions only meets with partial success for a camera body to be plastic molded, with a metal block used as an insert. In other words, in case a light alloy of such as aluminum is used to cast a camera body, there may be adopted large spaces or cavities in dies for the aforesaid interconnecting portions. In contrast thereto, where the body proper of the camera body is plastic-molded, with a metal block used an an insert, the film abutting surface of the metal block should be machined after the molding, so that the metal block is not entirely covered with the aforesaid interconnecting portions, and hence a portion of the metal block to be machined after molding is kept exposed. This necessarily results in a decrease in cross-sectional area of the interconnecting portions. As a result, when the molten plastic is poured through such a cavity in dies, which corresponds to one box-shaped portion, into a cavity corresponding to the other box-shaped portion, a smooth molten plastic flow cannot be achieved. Yet, molten plastic is cooled during its flowing through the cavities corresponding to the interconnecting portions, so that blow holes tend to be produced in the other box-shaped portion, thus lowering the strength thereof.
Yet further, the camera body as shown in the Japanese laid open patent publication No. Sho 51-6719 suffers from the drawback that a metallic block tends to be slid off, breaking the body proper when a large load is applied thereon upon treatment thereof after molding of the body proper if insertion of the metal block into the body proper is shallow. On the contrary thereto, if the insertion is deep so that the metal block is fully sandwiched by the body proper, then the thickness of the body proper inevitably increases, which results in bulkiness of the camera body.